Aeronautics is the science and practice of aircraft navigation. It is also used to refer to the engineering discipline related to the design, construction, and operation of aircraft. In relation to astronautics, aeronautics refers specifically to vehicles designed for travel within the atmosphere, while astronautics refers specifically to vehicles designed for travel outside of the atmosphere.
Aeronautics is sometimes divided into various disciplines. Aerostation is the design, construction, and operation of lighter-than-air vehicles such as balloons. Aviation is the design, construction, and operation of heavier-than-air vehicles such as airplanes and helicopters.
In common usage however, the term aviation is also used as a synonym for aeronautics, or sometimes even to refer to aeronautics and astronautics as a whole.
Early Aeronautics
One of the earliest scientists to study aeronautics was Leonardo DaVinci. DaVinci studied the flight of birds in developing engineering schematics for some of the earliest flying machines in the late fifteenth century AD.
Modern Aeronautics
Modern aeronautics is primarily conducted by independent corporations and universities. There are also a number of government agencies that study aeronautics, including NASA in the United States and the European Space Agency in Europe.
The term spaceship is generally applied only to spacecraft capable of transporting people.
A space suit has at times been called a miniature spacecraft or spaceship, emphasizing its purpose of keeping its wearer alive while traveling in the vacuum of outer space.
The spacecraft is one of the primal elements in science fiction. Numerous short stories and novels are built up around various ideas for spacecraft. Some hard science fictionbooks focus on the technical details of the craft, while others treat the spacecraft as a given and delve little into its actual implementation.
- [http://science.hq.nasa.gov/missions/phase.html NASA: Space Science Spacecraft Missions]
- [http://www.skyrocket.de/space/ Gunter's Space Page - Complete information on spacecraft]
- [http://www.cinespaceships.net/ Cinespaceships - Database on spaceships in movie]
- ja:宇宙船
Atmospheric reentry
Terminology, Definitions and Jargon
Over the decades since the late 1950s, a rich technical jargon has grown around the engineering of vehicles designed to enter planetary atmospheres. Definition of the jargon is prerequisite to meaningful discussion about atmospheric reentry.
Atmospheric entry is the transition from the vacuum of space to the atmosphere of any planet or other celestial body. The term is not used for landing on bodies which have no atmosphere such as the Moon.
Atmospheric reentry refers to the return to an atmosphere previously left for space. Often the word "atmospheric" is dropped and the term reentry (or re-entry) is taken to mean atmospheric reentry in context.
An entry vehicle is typically a non-military vehicle travelling from the near vacuum of space into the atmospere of a planet. An entry vehicle is used by National Aeronautics and Space Administration (NASA) and civilian space agencies for purposes of Space Exploration.
A Reentry Vehicle (RV) is a munition delivered by an Intercontinental Ballistic Missile (ICBM) of the United States Air Force (USAF) or the military of another country. The photographic film return capsule (now obsolete) for a low earth orbit satellite or military reconnaissance satellite was also called an "RV". It is a common error to call a non-military entry vehicle a "reentry vehicle".
A Reentry Body (RB) is a munition delivered by a Submarine Launched Ballistic Missile (SLBM) of the United States Navy (USN).
An RB or RV could in theory be the same device but never are (the USAF and USN never use the same design). The container inside an RB or RV holding the military payload (thermonuclear explosive) is called a bomb can.
The outer structure of an entry vehicle, RV or RB that defines its aerodynamics is called an aeroshell.
An entry vehicle, RV or RB is hypersonic when it has a supersonic free-stream velocity that creates a shock wave processing atmospheric gas into chemical dissociation, e.g. molecular nitrogen breaks down into atomic nitrogen. The gas layer processed by the shock wave located between the shockwave and aeroshell is called the shock layer.
The term hypersonics can have a special meaning refering to the engineering of vehicles that cruise or glide at hypersonic velocity or employ a supersonic combusion ramjet or scramjet.
Entry, Descent and Landing (EDL) is the process of getting an entry vehicle from orbit to a planet's surface. EDL includes parachute deployment (descent) and planet surface landing, e.g. braking rockets, air bags, etc.
Dynamic pressure is one half of the local density of the atmosphere times the atmosphere relative velocity squared. Dynamic pressure is typically referred to as "q".
A sphere-cone is a conical aeroshell with a spherical nose. The outer surfaces are tangential along the line of contact (ring) connecting the spherical nose to the cone or frustum. The outer surface is also called the outer mold line. The angle from the sphere-cone's axis of symmetry to the frustum is called the half-angle. The half-angle of a modern RV or RB is typically around 11 degrees. The half-angle of a non-military entry vehicle is typically 45 degrees or greater but never greater than 70 degrees.
The bluntness ratio of a sphere-cone is the ratio of the nose radius divided by the base radius. Most American interplanetary entry vehicles and the Mk-6 RV have a bluntness ratio of 1:2. The Mk-6 had a half angle of 20.5 deg. The Mk-6 carried the largest nuclear warhead of any ICBM RV, i.e. the W-53. The Mk-6 aeroshell is also the design ancestor of almost all American interplanetary entry vehicles, e.g. Pioneer Venus, Galileo Probe, etc.
A Thermal Protection System (TPS) is the atmospheric entry system or material used to protect an aeroshell's payload from heating due to hypersonic entry into the atmosphere. The outer layer of material on the aeroshell is also called TPS, TPS material or TPS layer. A famous TPS material is Teflon which was originally developed by the DuPont Corp. for use on RVs.
TPS failure occurs when the temperature of the material bonding the TPS layer to the aeroshell's structure exceeds the maximum allowed for the bonding material. A typical bondline material is RTV-560 (originally developed by General Electric for use on RVs). A typical maximum allowed bondline temperature is 250 deg.C. TPS failure can be exacerbated if spallation occurs. Spallation is where chunks of TPS material are torn away from the outer wall of the TPS. This can happen if the maximum allowed dynamic pressure is exceeded. After EDL or TPS failure, the entry vehicle is said to have augered in. This expression for undesired high speed surface impact dates back to World War II fighter pilots and was also used by test pilots at Edwards Air Force Base, the most well known being Brigadier General Chuck Yeager.
Angle-of-attack is the angle between an entry vehicle's principle axis (axis-of-symmetry) and the free stream velocity vector. There are two forms of angle-of-attack. The more common form used with aircraft restricts angle-of-attack to the aircraft's pitch plane. Side slip angle is used with this common form of angle-of-attack. The other form of angle-of-attack (typically used with entry vehicles) is not restricted to the pitch plane and is called total angle-of-attack. The parameter used with total angle-of-attack is aerodynamic roll angle. Angle-of-attack is often called "alpha". Side slip angle is normally called "beta". An aircraft or entry vehicle is at its trim angle-of-attack when its pitching moment is equal to zero, e.g. a pilot could let go of the control stick and the aircraft's attitude would remain unchanged.
Ballistic entry occurs when an entry vehicle has only drag with no apparent lift. An axisymmetric entry vehicle would have no apparent lift if its angle-of-attack time averaged out to zero, e.g. sinusoid angle-of-attack centered or trimmed about zero lift.
Lifting entry occurs when an entry-vehicle has lift and drag. A manned entry vehicle almost always uses lifting entry to reduce decelleration loading on the crew and improve cross range.
Ballistic coefficient is defined for an entry vehicle as its entry mass divided by the product of its aerodynamic area times its drag coefficient. For an RV or RB, the ballistic coefficient is the entry weight divided by the product of its aerodynamic area times its drag coefficient. Ballistic coefficient is sometimes called "beta".
Lift-over-drag ratio (L/D) is the ratio of the coefficient-of-lift divided by the coefficient-of-drag. The L/D of an entry vehicle undergoing ballistic entry is by definition zero.
The Mach number of an entry vehicle is a dimenionless number derived from the free stream relative velocity divided by the free stream speed-of-sound. Speed-of-sound is proportional to the square root of the absolute temperature of the gas. An extremely hot gas at high velocity could have a relatively low Mach number, e.g. the gas from a commercial plasma cutter flowing at orbital velocity (7.8 km/sec) could have a Mach number less than three. An entry vehicle is probably hypersonic if its Mach number is greater than 6 and certainly hypersonic if the the Mach number is greater than 9 (being hypersonic depends upon whether the shock layer has undergone chemical dissociation).
Entry angle, flight-path angle or velocity angle are different names for the angle of the velocity vector to the local horizon. Entry angle is typically referred to as "gamma". There are two types of gamma, i.e. relative gamma and inertial gamma. If no distinction is made then assume the gamma is relative.
A gas so tenuous that it can be modelled as a collection of tiny particles is called free molecular. The transition from a free molecular gas to a continuum gas can be determined by its Knudsen number.
Heat flux is the thermal power per unit area experienced by a TPS. The preferred units for heat flux are watts/cm² (note the mix of MKS and CGS units). Heat load is time integrated heat flux. Heat soak is the component of heat load that actually penetrates the TPS and entry vehicle structure.
The interval along the trajectory where the heat flux rises from insignificance, reaches its peak value and then descends back into insignificant is called the heat pulse. Heat pulse plotted as a function of time typically has a bell curve shape. The heat pulse for Mars Pathfinder lasted about 100 seconds. For the Galileo Probe, the heat pulse lasted about 70 seconds.
The total heat flux experienced by an aeroshell undergoing hypersonic entry can have up to three components, i.e convective heat flux, catalytic heat flux and radiative heat flux. Convective heat flux is simply heat convected from the hot shock layer gas to the cooler aeroshell wall. Catalytic heat flux is produced by dissociated gas species in the shocklayer gas recombining into less reactive molecules on the aeroshell wall thus releasing heat. Radiative heat flux comes from the intense light radiating from the shocklayer which is in a state of chemical nonequilibrium due to passing through the shock wave. As a function of time from entry, radiative heat flux always reaches its peak value before the convective heat flux reaches its peak value (this can be used as a simple test of a heating model's validity).
The stagnation point is a region of stagnant flow on the leading edge of an aeroshell. For a sphere-cone undergoing ballistic entry, the stagnation point is on the outer surface of the spherical nose that intersects with the sphere-cone's axis-of-symmetry. The region of high temperature, subsonic flow near the stagnation point is called the subsonic cap. For an aeroshell undergoing hypersonic heating from a laminar and convection dominated heat flux, the stagnation point is almost always the hotest point on the entry vehicle. Therefore conservative design for a TPS is normally based upon conditions at the stagnation point. However, for turbulent or radiation dominated heat flux, the stagnation point might not be the hottest point on the entry vehicle (this situation can occur during high speed return from the Moon or Mars).
The Fay-Riddell equation is a relatively compact closed form equation used to model the convective and catalytic heat flux at the stagnation point of an aeroshell. This equation was published in February 1958 by J. A. Fay and F. R. Riddell in the "Journal of the Aeronautical Sciences", Vol. 25, No. 2, page 73. The Fay-Riddell equation is remarkably accurate and sometimes used to validate modern computational fluid dynamics (CFD) solutions. Though virtually unknown outside the aerospace profession, the Fay-Riddell equation is amongst the most brilliant mathematical derivations in the history of science (comparable in mathematical sophistication to the solution of the Schrodinger equation for atomic hydrogen).
Newtonian impact theory is a method for modelling the aerodynamics of blunt entry vehicles at Mach numbers that are normally hypersonic. Newtonian impact theory enables closed form solutions for simple aeroshell geometries and was a preferred modelling method prior to the development of CFD. Newtonian impact theory is still extremely useful for the preliminary design of entry vehicles.
Aerobraking occurs when the free molecular gas of a planet's upper atmosphere is used to reshape the orbit of a spacecraft. Heating due to aerobraking is normally insignificant thus requiring no special thermal protection. The term "aerobraking" is often used incorrectly by people outside the aerospace profession.
Aerocapture occurs when the continuum gas of a planet's atmosphere is used to dissipate the kinetic energy of a spacecraft entering from a heliocentric hyperbolic trajectory and then skipping out of the atmosphere into an elliptical orbit centered around the capturing planet. Aerocapture is an enabling technology for Mars exploration. Protecting against heat soak resulting from aerocapture is technically challenging and maybe insolvable for aerocapture to a gas giant planet such as Neptune.
Skip reentry is aerocapture to a suborbital ellipse having an apoapsis (maximum orbital altitude) that is just outside of the atmosphere.
The maximum allowed entry angle for an entry-vehicle is called the overshoot angle. The minimum allowed entry angle for an entry-vehicle is called the undershoot angle. The angular range between the overshoot and undershoot angles defines the entry corridor.
A well posed aerocapture trajectory has two possible trajectory solutions to one capture orbit apoapsis. The "lift-down" trajectory (the bank angle points the lift vector towards the ground) is also called an overshoot trajectory. The "lift-up" trajectory (the bank angle points the lift vector towards the zenith) is also called an undershoot trajectory.
A reaction control system (RCS) is the system of small rocket thrusters that reorient a spacecraft with respect to the inertial frame of reference. Control of an RCS is based upon an inertial measurement unit (IMU).
A three degree of freedom (3-DoF) trajectory simulation is a model of an entry vehicle's center-of-mass trajectory based upon three spatial coordinates, e.g. X,Y,Z and their corresponding velocities. A six degree of freedom (6-DoF) trajectory simulation is a 3-DoF simulation augmented by a model of the vehicle's orientation based upon pitch, yaw and roll angles along with Euler angles. A 3-DoF simulation treats angle-of-attack and bank angle as user input parameters. A 6-DoF simulation includes angle-of-attack and bank angle within the model (control surface orientation and RCS are the user input parameters). 6-DoF simulation of a hypersonic entry vehicle is much more complicated than classical 6-DoF simulation of a low speed, low flying airplane.
The aerodynamic stability of an aeroshell has two aspects, i.e. static stability and dynamic stability. An aeroshell has static stability when its center of mass is upstream from its aerodynamic center. An aeroshell with static stability can still be unstable if it lacks dynamic stability. Dynamic stability is determined through 6-DoF trajectory simulation.
Blunt Body Entry Vehicles
center of mass
These four shadowgraph images represent early re-entry vehicle concepts. A shadowgraph is a process that makes visible the disturbances that occur in a fluid flow at high velocity, in which light passing through a flowing fluid is refracted by the density gradients in the fluid resulting in bright and dark areas on a screen placed behind the fluid.
H. Julian Allen and A. J. Eggers, Jr. of the National Advisory Committee for Aeronautics (NACA) made the counter-intuitive discovery in 1952 that a blunt shape (high drag) made the most effective heat shield. From simple engineering principles, Allen and Eggers showed that the heat load experience by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater than drag, the less the heat load. Their discovery was initially treated as a military secret but eventually published in 1958 as "A Study of the Motion and Aerodynamic Heating of Ballistic Missiles Entering the Earth's Atmosphere at High Supersonic Speeds," NACA Report 1381. The Blunt Body Theory made possible the heat shield designs that were embodied in the Mercury, Gemini and Apollo space capsules, enabling astronauts to survive the fiery re-entry into Earth's atmosphere.
Entry Vehicle Shapes
There are several basic shapes used in designing entry vehicles:
The simplist axisymmetric shape is the spherical section. The spherical section's aerodynamics are easy to model analytically using Newtonian impact theory. Likewise, the spherical section's heat flux can be accurately modelled with the Fay-Riddell equation. The static stability of a spherical section is assured if the vehicle's center-of-mass is upstream from the center-of-curvature (dynamic stability is more problematic). By flying at an angle-of-attack, a spherical section has modest aerodynamic lift thus providing some cross-range capability and widening its entry corridor. In the late 1950s and early 1960s, high speed computers were not yet available and CFD was still embryonic. Because the spherical section was susceptible to closed form analysis, that geometry became the default for conservative design. Consequently, manned capsules of that era were based upon the spherical section. The most famous example of a spherical section entry vehicle was the Apollo Command Module (Apollo-CM). Other examples of spherical section entry vehicles as manned capsules are Soyuz/Zond, Vostok, Gemini and Mercury.
The sphere-cone is a spherical section with a frustum attached. The sphere-cone's dynamic stability is typically better than a spherical section. With a sufficiently small half-angle and properly placed center-of-mass, a sphere-cone can provide aerodynamic stability from atmospheric entry to surface impact. The sphere-cone shape was developed early in the evolution of ICBM RV design, e.g. the Mk-6. The sphere-cone is the preferred geometry for modern ICBM RVs. Reconnaissance satellite RVs also used a sphere-cone shape and were the first example of a non-munition entry vehicle (Discoverer-I, launched on 28 February 1959). The sphere-cone was latter used for space exploration missions to other celestial bodies or for return from open space, e.g. Stardust probe. Space exploration entry vehicles based upon the sphere-cone have landed on the surface or entered the atmospheres of Mars, Venus, Jupiter and Titan.
The biconic is a sphere-cone with an additional frustum attached. The biconic offers a significantly improved L/D ratio. A biconic designed for Mars aerocapture typically has an L/D of approximately 1.0 compared to an L/D of 0.37 for the Apollo-CM. The higher L/D makes a biconic shape better suited for transporting people to Mars due to the lower peak deceleration. Arguably, the most significant biconic ever flown was the Advanced Maneuverable Reentry Vehicle (AMaRV). Four AMaRVs were made by the McDonnell-Douglas Corp and represented a quantum leap in RV sophistication. Three of the AMaRVs were launched by Minuteman-1 ICBMs on 20 December 1979, 8 October 1980 and 4 October 1981. AMaRV had an entry mass of approximately 470 kg, a nose radius of 2.34 cm, a forward frustum half angle of 10.4 deg., an inter-frustum radius of 14.6 cm, aft frustum half angle of 6 deg, and an axial length of 2.079 meters. No accurate diagram or picture of AMaRV has ever appeared in the open literature. A schematic sketch does appear in "Dynamics of Atmospheric Re-Entry" by Regan and Anadakrishnan along with trajectory plots showing hairpin turns. AMaRV's attitude was controlled through a split body flap (also called a "split-windward flap") along with two yaw flaps mounted on the vehicle's sides. Hydraulic actuation was used for controlling the flaps. AMaRV was guided by a fully autonomous navigation system designed for evading anti-ballistic missile (ABM) interception. The McDonnell Douglas DC-X (also a biconic) was essentially a scaled up version of AMaRV. AMaRV and the DC-X also served as the basis for an unsuccessful proposal for what eventually became the Lockheed Martin X-33. Amongst aerospace engineers, AMaRV has achieved legendary status along side such technological marvels as the SR-71 Blackbird and the N1 rocket.
Non-axisymmetric shapes have been used for manned entry vehicles. One example is the winged orbit vehicle that uses a delta wing for maneuvering during descent much like a conventional glider. This approach has been used by the American Space Shuttle and the Soviet Buran. The lifting body is another entry vehicle geometry and was used with the X-23 PRIME (Precision Recovery Including Manoeuvring Entry) vehicle.
The proposed MOOSE system would have used a one-man inflatable ballistic capsule as an emergency astronaut entry vehicle.
Ablative Heat Shields
The type of heat shield that best protects against high heat flux is the ablative heat shield. The ablative heat shield functions by lifting the hot shock layer gas away from the heat shield's outer wall (creating a cooler boundary layer) through blowing. This process of reducing the heat flux experienced by the heat shield's outer wall is called blockage. Ablation causes the TPS layer to char, melt, and sublimate through the process of pyrolysis. The gas produced by pyrolysis is what drives blowing and causes blockage of convective and catalytic heat flux. Ablation can also provide blockage against radiative heat flux by introducing carbon into the shock layer thus making it optically opaque. Radiative heat flux blockage was the primary thermal protection mechanism of the Galileo Probe TPS material (carbon phenolic). Thermal protection can also be enhanced in some TPS materials through coking. Coking is the process of forming solid carbon on the outer char layer of the TPS. TPS coking was discovered accidently during development of the Apollo-CM TPS material (Avcoat 5026-39).
The thermal conductivity of a TPS material is proportional to the material's density. Carbon phenolic is a very effective ablative material but also has high density which is undesireable. If the heat flux experienced by an entry vehicle is insufficient to cause pyrolysis then the TPS material's conductivity could allow heat flux conduction into the TPS bondline material thus leading to TPS failure. Consequently for entry trajectories causing lower heat flux, carbon phenolic is inappropriate and lower density TPS materials like SLA-561V or SIRCA are better design choices. SLA stands for "Super Light weight Ablator". All of the 70 deg. sphere-cone entry vehicles sent by NASA to Mars used SLA-561V as their primary TPS material. SLA-561V begins significant ablation at a heat flux of approximately 80 watts/cm² but will fail for heat fluxes greater than 300 watts/cm². SLA-561V would be unusable as an Apollo-CM TPS material for lunar return where the peak heat flux is around 497 watts/cm². The peak heat flux experienced by the Viking-1 aeroshell which landed on Mars was 21 watts/cm². For Viking-1, the TPS acted as a pure thermal insulator and never experienced significant ablation (an inappropriate design choice). However for the Mars Pathfinder aeroshell, the peak heat flux was 106 watts/cm². SLA-561V was an appropriate design choice for Mars Pathfinder.
Thermal Soak Heat Shields
Thermal soak is a part of almost all TPS schemes. For example, an ablative heat shield loses most of its thermal protection effectiveness when the outer wall temperature drops below the minimum necessary for pyrolysis. From that time to the end of the heat pulse, heat from the shock layer soaks into the heat shield's outer wall and would eventually convect to the payload. This outcome is prevented by ejecting the heat shield (with its soaked heat) prior to the heat reaching the inner wall.
Mars Pathfinder
Thermal soak TPS is intended to shield mainly against head load and not against a high peak heat flux (a long duration heat pulse of low intensity is assumed). The Space Shuttle orbit vehicle was designed with a reusable heat shield based upon a thermal soak TPS. A Space Shuttle's underside is coated with thousands of tiles made of silica foam that are intended to survive multiple reentries with only minor repairs between missions. When a Space Shuttle lands, there is a significant amount of heat stored in the TPS. Shortly after landing, a ground support cooling unit connects to the orbiter's internal freon coolant loop to remove heat soaked in the TPS and orbiter structure.
Typical Space Shuttle's TPS tiles (LI-900) have remarkable thermal protection properties but are relatively brittle and break easily. An LI-900 tile can be exposed to a temperature of thousands of degrees on one side, but merely warm to the touch on the other side. An impressive stunt that can be performed with a cube of LI-900 is to remove it glowing white hot from a furnace and then hold it with one's bare fingers without discomfort along the cube's edges (the author has done this).
Passively Cooled Heat Shields
In some early ballistic missile RVs and the sub-orbitalMercury spacecraft, radiatively cooled TPS were used to initially store heat flux during the heat pulse and then latter radiate the heat away from the vehicle. However the technique required a considerable quantity of metal TPS, adding greatly to the vehicle's mass. Consequently ablative or thermal soak TPS are now more common.
Some high-velocity aircraft, such as the SR-71 Blackbird and Concorde, have to deal with heating similar to that suffered by spacecraft but with lower intensity. Shockwaves can attach to the pointed nose and heat the aircraft through wave drag. Generally heat is conducted through the aluminium or titaniumalloy, or occasionally stainless steel skins. In the case of Concorde the nose is permitted to reach a maximum operating temperature of 260 degrees Fahrenheit (127 degrees Celsius), typically 180 degrees Celsius (325 degrees Fahrenheit) warmer than the external air.
Actively Cooled Heat Shields
Various advanced reusable spacecraft and hypersonic aircraft designs have been proposed to employ heat shields made from temperature-resistant metalalloys that incorporated a refrigerant or cryogenic fuel circulating through them. Such a TPS concept was proposed for the X-30 National Aerospace Plane (NASP). The NASP was supposed to have been a scramjet powered hypersonic aircraft but failed in development.
In the early 1960s various TPS systems were proposed to use water or other cooling liquid sprayed into the shock layer. Such concepts never got past the proposal phase since ordinary ablative TPS is much more reliable and mass efficient.
Feathered Reentry
In 2004, aircraft designer Burt Rutan demonstrated the feasibility of an alternative or complementary approach to atmospheric reentry with the suborbital SpaceShipOne.
SpaceShipOne has what has been described as a pair of flipping wings; the spacecraft itself changes shape for reentry.
This increases drag, as the craft is now less streamlined. This results in more atmospheric gas particles hitting the spacecraft at higher altitudes than otherwise. The aircraft thus slows down more in higher atmospheric layers (which is the very key to efficient reentry, see above). It should also be noted that SpaceShipOne, in its "wings flipped" configuration, will automatically orient itself to a high drag attitude. Rutan has compared this to a falling shuttlecock.
However, it is important to realise that the velocity obtained by SpaceShipOne prior to reentry is much lower than of an orbital spacecraft, and most engineers (including Rutan) do not consider the shuttlecock reentry technique viable for return from orbit.
Entry Vehicle Design Considerations
There are four critical parameters considered when designing a vehicle for atmospheric entry:
# Peak heat flux
# Heat load
# Peak deceleration
# Peak dynamic pressure
Peak heat flux selects the TPS material. Heat load selects the thickness of the TPS material stack. Peak deceleration is of major importance for manned missions. The upper limit for manned return to Earth from Low Earth Orbit (LEO) or lunar return is 10 G. For martian atmospheric entry after long exposure to zero gravity, the upper limit is 4 G. Peak dynamic pressure can also influence the selection of the outermost TPS material if spallation is an issue.
The designer typically considers two worst case trajectories, the undershoot and overshoot trajectories. The overshoot trajectory is typically defined as the shallowest allowable entry velocity angle prior to atmospheric skip-off. The overshoot trajectory has the highest heat load and sets the TPS thickness. The undershoot trajectory is defined by the steepest allowable trajectory. For manned missions the steepest entry angle is limited by the peak deceleration. The undershoot trajectory has the highest heat flux and therefore defines selection of the TPS material.
History's Most Difficult Atmospheric Entry
shuttlecockshuttlecock
The highest speed controlled entry so far achieved was by the Galileo Probe. The Galileo Probe was a sphere-cone that entered Jupiter's atmosphere at 47.4 km/sec (atmosphere relative speed at 450 km above the 1 bar reference altitude). The peak (blocked) total heat flux for the Galileo Probe was 16900 watts/cm². By way of comparison, the peak total heat flux experienced by a Martian entry vehicle (Mars Pathfinder) was 106 watts/cm². The Apollo-4 (AS-501) command module which reentered the Earth's atmosphere at a velocity of 10.77 km/sec (atmosphere relative speed at 121.9 km altitude) experienced a peak total heat flux of 497 watts/cm². Carbon phenolic was the TPS material used for the Galileo Probe. Carbon phenolic was earlier used for the Pioneer Venus probes. Carbon phenolic was originally developed as a rocket nozzle throat material and then latter used for RV nose tips. The Galileo Probe experienced far greater TPS recession near the base of its frustum than expected. Despite a factor of two safety factor in TPS thickness, the Galileo Probe's heatshield almost failed. The two conflicting theories describing this near failure were higher than expected spallation due to hypersonic turbulence and/or radiative heat flux. The precise mechanism for this higher TPS recession is unknown and currently beyond definitive theoretical analysis.
Notable Atmospheric Entry Mishaps
- Vostok 1 - The service module failed to detach for 10 minutes, but the crew survived.
- Mercury 6 - Instrument readings show that the heat shield and landing bag were not locked. It is decided to left the retrorocket in position during reentry. Astronaut John Glenn survived.
- Voskhod 2 - The service module failed to detach for some time, but the crew survived.
- Soyuz 1 - Different accounts exist. Either the attitude control system failed while still in orbit and/or parachutes got entangled during the landing sequence (EDL failure). Cosmonaut Vladimir Mikhailovich Komarov died.
- Soyuz 5 - The service module failed to detach, but crew survived.
- Soyuz 11 - The crew perished due to early depressurization.
- Space Shuttle Columbia - The failure of a Reinforced Carbon-Carbon (RCC) tile on a wing leading edge led to breakup of the orbit vehicle at hypersonic speed resulting in the loss of all seven crew members.
- Mars Polar Lander (MPL) - Failed during EDL. The failure was believed to be the consequence of a software error. The precise cause is unknown due to lack of real time telemetry.
- Genesis - The parachute failed to deploy due to a G-switch being installed backwards (a similar error delayed parachute deployment for the Galileo Probe). Consequently, the Genesis entry vehicle augered into the desert floor. The payload was damaged but it was later claimed that some science data was recoverable.
Uncontrolled reentry
More than 100 metric tons of man-made objects reenter in an uncontrolled fashion each year. The vast majority burn up before reaching earth's surface. On average, about one cataloged object reenters per day. Approximately one-fourth of all objects are of U.S. origin. Due to the Earth's surface being primarily water, most objects that survive re-entry land in one of the world's oceans.
Reference Books
Martin, John J., "Atmospheric Entry - An Introduction to Its Science ane Engineering," Prentice-Hall, Old Tappan, NJ, (1966).
Regan, Frank J., "Re-Entry Vehicle Dynamics," AIAA Education Series, American Institute of Aeronautics and Astronautics, Inc., New York, ISBN 0-915928-78-7, (1984).
Regan, Frank J. and Anadakrishnan, Satya M., "Dynamics of Atmospheric Re-Entry," AIAA Education Series, American Institute of Aeronautics and Astronautics, Inc., New York, ISBN 1-56347-048-9, (1993).
Etkin, Bernard, "Dynamics of Atmospheric Flight," John Wiley & Sons, Inc., New York, ISBN
0-471-24620-4, (1972).
Commentary about the Reference Books
John J. Martin's book was the first and arguably the best in the open literature about designing reentry vehicles. In his book, Martin showed an incredible depth and breadth of knowledge. Unfortunately, this book has been out-of-print for decades but can still be easily acquired second hand through the Internet and is not overly expensive.
John J. Martin was educated as a mechanical engineer, receiving a Ph.D. from Purdue University in 1951. He joined North American Aviation in 1951 and moved to the Bendix Corp. in 1953. In 1960 he joined the Institute for Defense Analyses. While on sabbatical at the Royal Aircraft Establishment in Farnborough, England, Martin wrote "Atmospheric Entry". Sir Michael James Lighthill, who was Martin's host at the Royal Aircraft Establishment, wrote the Foreward to Martin's book. In 1969, Martin served as a science advisor to the US President. During 1973-1974 Martin served as an Associate Deputy Director at the Central Intelligence Agency and later as Deputy Assistant Secretary of the US Air Force. In 1984, Martin became an Associate Administrator at NASA.
"Dynamics of Atmospheric Re-Entry" by Frank J. Regan and Satya M. Anandakrishnan is a revision of Regan's earlier book, "Re-Entry Vehicle Dynamics". Unfortunately Chapter 10 of "Re-Entry Vehicle Dyanmics" was deleted when the book was revised into the newer version. Chapter 10, titled "Moment Equations in Constant Density Atmosphere" concerned the subjects of entry vehicle roll resonance and tricyclic theory. "Re-Entry Vehicle Dynamics" has been out-of-print for years and currently no used copies are listed on the Internet. If you find a second hand copy of "Re-Entry Vehicle Dynamics", buy it (it's a very rare book). Should you find "Re-Entry Vehicle Dynamics" in a library, photocopy Chapter 10. Despite the omissions from the earlier version, "Dynamics of Atmospheric Re-Entry" is a very useful book and still in print, though very expensive (current list price of $105.95).
Bernard Etkin is the world recognized authority on aircraft guidance and control. Classical 6-DoF theory for aircraft assumes a flat earth with constant atmospheric density in an inertial frame. Consequently classical 6-DoF theory should not be used for simulating hypersonic atmospheric flight that lasts for several minutes. Clasical 6-DoF for hypersonic flight is approximately correct only for a few seconds, e.g. stability analysis for a time discrete event. Etkin's treatment of 6-DoF theory in "Dynamics of Atmospheric Flight" was unusual in being sufficiently general that it touched upon hypersonic flight.
A basic dilemma with aerospace technological information is its intrinsic dual use, i.e. aerospace technology can be used towards the extreme benefit of humanity or towards its utter detriment. The technology of atmospheric entry owes its origins to the development of ballistic missiles during the Cold War. Given the enormous expense required in developing atmospheric entry technology, it is doubtful this technology could ever have appeared without the military incentive. It is profoundly ironic that the same techology enabling the annihilation of modern civilization also enabled the exploration and development of outer space. Mankind's survival beyond its planet of origin is dependent upon atmospheric entry technology. Therein lies the fundamental dilemma, aerospace technology must remain open to enable mankind's access to outer space while also remaining restricted to prevent proliferation of weapons of mass destruction (WMD). This basic dilemma is present throughout the literature on atmospheric entry. There is a glass wall between pedagogical and practical information. For example, in the text books listed above, a topic thread will proceed as long as the information is nonspecific but almost always stops at the point of practical application. To go beyond pedagogical information, one must search the technical literature (NASA Technical Reports, declassified technical reports and peer reviewed archive literature). Declassified technical reports are a frustrating information source since many of the reports were destroyed prior to going through the legally required declassification process. It is almost always true that significant documents referred to in declassified technical reports no longer exist (technical information that cost many millions of dollars to develop has vanished without a trace).
- [http://www.chrisvalentines.com/sts107/reentrypage.html Space Shuttle Columbia Final Reentry Video]
- [http://www.chrisvalentines.com/sts107/realtime.html Columbia Reentry Video Reconstruction, California to Texas]
ja:大気圏再突入
Maria Dombrowska
Maria Dąbrowska (født 6. oktober1889, død 1965) var en polsk forfatterinde. På dansk skrives hendes navn til tider Maria Dombrowska. I Danmark er hun mest kendt for sin store slægtsroman, Nætter og dage (polskNoce i dnie, filmatiseret).
D
موجودات فضایی
این مسئله که آیا حیات آلی در جای دیگری از جهان وجود دارد و اینکه آیا ممکن است تمدن ماوراء زمینی وجود داشته باشد، موضوع مهمی برای دانشمندان و عموم مردم بوده است. با وجود چنین واقعیتی که هیچ موجود زندهای در خارج از سیاره ما کشف نشده، علوم طبیعی جدید به قدری پیشرفت نمودهاند که اکنون میتوان ا
کهریزک
کَهریزک نام یکی از روستاهای پیرامون شهر تهران است که امروزه جزئی از محدوده شهری تهران بشمار میآید.
جستارهای وابسته
- کهریزک بنا شده است.
این آسایشگاه در سال 1351 به همت دکتر محمد رضا حکیم زاده، که در آن زمان مدیر بیمارستان فیروزآبادی شهر ری بود، فعالیت خود را آغاز کرد. این محل که در ابتدا از فضایی کوچک و محدود
سونوگرافی
سونوگرافی فراصوتی یکی از روشهای تشخیص بیماری در پزشکی است. به این روش اکوگرافی، پژواکنگاری و صوتنگاری هم گفته شده.
ریشه لغوی
کلمه سونوگرافی از لفظ لاتین sono به معنی صوت و نیز graphic به معنی شکل و ترسیم گرفته شده و ultrasound از ultra به معنی ماورا و نیز sound به معنی صوت یا صدا گرفته شده است.
سونوگرافی فراصوتی
سونوگرافی فراصوتی یکی از روشهای تشخیص بیماری در پزشکی است. به این روش اکوگرافی، پژواکنگاری و صوتنگاری هم گفته شده.
ریشه لغوی
کلمه سونوگرافی از لفظ لاتین sono به معنی صوت و نیز graphic به معنی شکل و ترسیم گرفته شده و ultrasound از ultra به معنی ماورا و نیز sound به معنی صوت یا صدا گرفته شده است.
ذراع
ذراع " بازو " یا آرنج، واحد طول ( درازی ) وآن عبارت است از ابتدای ساعد است " مرفق " تاسر انگشتان.
طول معادل شش قبضه ( مشت ) که با انگشتان ( غیر انگشت نشست ) با یگدیگر متصل ساخته ملاحظه نمایند.
واین مجموع بقدر بیست وچهار انگشت خواهد بود که از جانب پهنا بیگدیگر گذارند.
از این گونه واحد طول در ساخت سد ، آلمان:
- بن - هامبورگ - هانوفر - مونیخ - بایرویت (باواریای شمالی)
